Thermodynamically stable vesicle formation from glycolipid biosurfactant sponge phase

Thermodynamically stable vesicle (L α1) formation from glycolipid biosurfactant sponge phase (L 3) and its mechanism were investigated using a “natural” biocompatible mannosyl-erythritol lipid-A (MEL-A)/ l-α-dilauroylphosphatidylcholine (DLPC) mixture by varying the composition. The trapping efficiency for calcein and turbidity measurements clearly indicated the existence of three regions: while the trapping efficiencies of the mixed MEL-A/DLPC assemblies at the compositions with X DLPC ≤ 0.1 or X DLPC ≥ 0.8 were almost zero, the mixed assemblies at the compositions with 0.1 < X DLPC < 0.8 had some trapping efficiency, and in particular the composition with X DLPC = 0.3 exhibited the maximum trapping efficiency. Confocal laser scanning microscopy (CLSM) and freeze–fracture electron microscopy (FFEM) determined that the assemblies in the compositions with X DLPC ≤ 0.1 were droplets with a sponge phase (L 3) with diameter from 2 to 20 μm and those of X DLPC ≥ 0.8 were multilamellar vesicles (L α) with diameter from 2 to 10 μm. Meanwhile, dynamic light scattering (DLS) measurement revealed that the average size of the vesicles at the composition of X DLPC = 0.3 was 633.2 nm, which is remarkably small compared to other compositions. Moreover, the mixed vesicle solution at the composition of X DLPC = 0.3 was slightly bluish and turbid and kept its dispersion stability at 25 °C for more than 3 months, indicating the formation of a thermodynamically stable vesicle (L α1). These results exhibited the formation of a thermodynamically stable vesicle (L α1) with a high dispersibility from the MEL-A/DLPC mixture. The asymmetric distribution of MEL-A and DLPC in the two vesicle monolayers caused by the difference in geometrical structures is very likely to have changed their self-assembled structure from a sponge phase (L 3) to a thermodynamically stable vesicle (L α1).